The banking group behind NatWest's latest IT collapse has promised to compensate customers who were left out of pocket as a result of being blocked from accessing their cash.

Furious customers have deluged the bank with complaints about its second computer hitch to happen within a year, which has also affected sister banks Royal Bank of Scotland (RBS) and Ulster Bank.

RBS Group has apologised for the "unacceptable failure", which it said was due to a hardware fault unrelated to last summer's meltdown that cost the group £175 million to put right.

Problems from around 9pm on Wednesday meant that customers were barred from withdrawing money at ATMs, with reports of some having their cards "swallowed" at cashpoints. Online and telephone banking services were also disrupted.

The bank said that the hitch was cleared up a couple of hours later, but some customers were still reporting problems on NatWest's website on Thursday. The group is urging customers who have been left worse off to get in touch and promised they will not be permanently out of pocket.

A statement from RBS Group said: "This problem was caused by a hardware fault and was not related to the issues we experienced last summer. It was much easier to fix, though clearly an unacceptable failure. Any customer who was left out of pocket due to this outage should get in touch so we can put things right for them."

A spokeswoman for the group said claims will be looked at on a "case by case basis" to work out if people should be compensated for any extra costs. Customers will not be automatically entitled to a payout just because they had problems and those who believe they are entitled to some cash should keep evidence such as receipts.

The BBC quoted NatWest customer Darren Reuben who said he had to get someone else to cover a restaurant bill on Wednesday evening. Mr Reuben said he was given £70 in compensation for the embarrassment caused after turning down an offer of £30. He said: "It was paid into my account straight away. While it was embarrassing, they did me a good turn."

It is unclear exactly how many of the 17 million personal banking customers of NatWest, RBS and Ulster Bank had been affected by the problems, although many people vented their frustration online. One Twitter user said: "Natwest, you left me with no dinner tonight, and left me walking home in the rain!"

In its recently-released annual report, RBS said it had been investing in technology to make it easier for people to bank with the group, including introducing more than 200 ATMs and cash deposit machines.